i88 
Mr. Home on the Gizzards 
with little more than the food between them; the hard sub- 
stances interposed, being only of a small size, so as not to 
interfere with their regular correspondence with each other. 
In these gizzards there is not the rotatory motion mentioned 
in the turkey, but a regular sliding motion, begun by the 
strong muscle forcing one side of the horny substance over 
the surface of the other ; and it is brought back by the action 
of the weaker. 
This motion bears so great a resemblance to that of the 
grinding teeth of ruminating animals, in which the teeth of 
the under jaw slide upwards, within those of the upper, 
pressing the food between them, and fitting it by this peculiar 
kind of trituration for being digested ; that it is fair to conclude, 
it is for the same purpose, more especially, when we only 
find it in those birds that graze. 
The gizzard of the goose is evidently fitted for a harsher 
kind of grass than that of the swan, and we find that the 
goose prefers the common grass of the fields, while the swan 
is partial to the soft weed found in ponds and rivers, and 
only grazes occasionally. 
The bill of the goose breaks off the grass short from the 
ground, nearly in the same manner as the fore teeth of rumi- 
nants. In like manner as ruminants have only teeth in the 
under jaw, so has the goose small pointed cuticular teeth in 
the lower portion of the bill, which, when the bill is closed, 
fit into grooves in the upper one, so that the grass inclosed 
in the space between the teeth and grooves, is nipped off. 
There is a swell in the lower part of the oesophagus in 
these birds, which is peculiar to them : this answers the pur- 
pose of a reservoir, in which the grass is retained, macerated. 
